Compact structure of gas sensor and production method thereof

ABSTRACT

An improved structure of a gas sensor is provided. The gas sensor includes a hollow housing in which a gas sensing element is disposed, a protective cover installed on an end of the housing, a metallic cover installed on the other end of the housing, and an insulator in which end portions of leads connecting with the gas sensing element are disposed. The insulator is retained at a flange thereof in the housing elastically using a washer. The protective cover is welded at an end thereof to the whole circumference of the end of the housing.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field of the Invention The present invention relatesgenerally to a gas sensor which may be employed in an air-fuel ratiocontrol system for automotive vehicles for measuring the concentrationof gas such as O₂, NOx, or CO, and more particularly to an improvedcompact structure of such a gas sensor and a production method thereof.

[0002] 2. Background Art

[0003] It is known that control of burning of an internal combustionengines as a function of the concentration of oxygen contained inexhaust gasses and the air-fuel ratio of a mixture is effective in theenergy saving and the emission control. As one of gas sensors suitablefor measuring the concentration of oxygen in exhaust gasses, a gassensor including a solid electrolyte body made of zirconia is known.This type of gas sensor is, however, required to be reduced in size andproduction costs and improved in durability and reliability. Theapproach to satisfaction of these requirements has still left room forimprovement.

SUMMARY OF THE INVENTION

[0004] It is therefore a principal object of the present invention toprovide a gas sensor designed to meet the above discussed requirementsin a conventional structure of a gas sensor.

[0005] According to one aspect of the invention, there is provided a gassensor which comprises: (a) a gas sensing element having an gas-exposedportion; (b) a hollow housing having a first and a second end, thehousing holding the gas sensing element therein so as to arrange thegas-exposed portion outside the housing for exposure to a gas to bemeasured; (c) a protective cover installed on the first end of thehousing to cover the gas-exposed portion of the gas sensing element; (d)leads connecting with the gas sensing element, extending from the secondend of the housing for electrical communication with an external device;(e) a metallic cover installed on the second end of the housing, themetallic cover including a small-diameter portion, a large-diameterportion, and a shoulder portion connecting the smaller-diameter portionand the large-diameter portion; and (f) an insulator including a bodyand a flange projecting from the body, having disposed therein the leadsconnecting with the gas sensing element, the body having an outerdiameter smaller than an inner diameter of the small-diameter portion ofthe metallic cover and being disposed within the small-diameter portion,the flange having a first and a second end surface opposed to each otherand an outer diameter which is smaller than an inner diameter of thelarge-diameter portion of the metallic cover and which is greater thanthe inner diameter of the small-diameter portion, the insulator beingdisposed in the metallic cover with the first end surface of the flangeurged by an elastic member to bring the second end surface into constantengagement with an inner wall of the shoulder portion of the metalliccover.

[0006] In the preferred mode of the invention, the metallic cover has agiven length. The elastic member is so designed as to produce a firstpressure acting on an inner wall of the large-diameter portion of themetallic cover in a radial direction of the large-diameter portion and asecond pressure acting on the second end surface of the flange of theinsulator in a lengthwise direction of the metallic cover perpendicularto the radial direction of the metallic cover.

[0007] The elastic member includes an annular plate and tabs. Theannular plate has a diameter smaller than the inner diameter of thelarge-diameter portion of the metallic cover. The tabs projects from theannular plate so as to establish elastic engagement with the inner wallof the large-diameter portion of the metallic cover.

[0008] The elastic member also include guide protrusions each of whichis disposed between adjacent two of the tabs and which projects from theannular plate to a circular line smaller than the inner diameter of thelarge-diameter portion of the metallic cover.

[0009] The elastic member may be made of a plate member having opposedsurfaces which are symmetrical with each other.

[0010] The tabs of the elastic member may be so designed that when theinsulator is inserted into the metallic cover, some of the tabs are bentelastically in a first direction away from one of the opposed surfacesof the annular plate by elastic pressure produced by the insertion ofthe insulator, while the other tabs are bent elastically in a seconddirection opposite the first direction.

[0011] The tabs may extend from the annular plate at an angle ofapproximately 45° or more to one of the opposed surfaces of the annularplate.

[0012] The elastic member may also include a protrusion formed on one ofthe opposed surfaces thereof.

[0013] At least one of the opposed surfaces of the elastic member ispainted so that the opposed surfaces have different colors.

[0014] The tabs of the elastic member may alternatively be so designedthat when the insulator is inserted into the metallic cover, the tabsare bent elastically in the same direction away from one of the opposedsurfaces of the annular plate by the elastic pressure produced by theinsertion of the insulator.

[0015] An elastic insulating member is disposed on an end of themetallic cover remote from the housing to retain the leads therein. Ifan outer diameter of the elastic insulating member is defined as E, andan outer diameter of the insulator is defined as F, then E≧F.

[0016] According to the second aspect of the invention, there isprovided a gas sensor which comprises: (a) a gas sensing element havingan gas-exposed portion; (b) a hollow housing having a first and a secondend, the housing holding the gas sensing element therein so as toarrange the gas-exposed portion outside the housing for exposure to agas to be measured; (c) a protective cover installed on the first end ofthe housing to cover the gas-exposed portion of the gas sensing element;(d) leads connecting with the gas sensing element, extending from thesecond end of the housing for electrical communication with an externaldevice; (e) an insulator retaining therein the leads connecting with thegas sensing element; and (f) a metallic cover joined directly to thehousing to hold the insulator therein.

[0017] In the preferred mode of the invention, the housing has an outerwall extending between the first and second ends. The metallic cover iswelded to the whole of a circumference of the outer wall of the housing.

[0018] A welded portion is formed with welding of the metallic cover andthe housing which includes a wider portion formed in the metallic coverand a narrower portion formed in the outer wall of the housing. Ifmaximum widths of the wider and narrower portions are defined as A andB, respectively, the depth of the narrower portion is defined as D, andthe thickness of the metallic cover 3 defined as T, conditions of B≧0.6Aand D≧T are satisfied.

[0019] An elastic insulating member is disposed on an end of themetallic cover remote from the housing to retain the leads therein. Ifan outer diameter of the elastic insulating member is defined as E, andan outer diameter of the insulator is defined as F, then E≧F.

[0020] According to the third aspect of the invention, there is provideda gas sensor which comprises: (a) a gas sensing element having angas-exposed portion; (b) a hollow housing having a first and a secondend, the housing holding the gas sensing element therein so as toarrange the gas-exposed portion outside the housing for exposure to agas to be measured; (c) a protective cover installed on the first end ofthe housing to cover the gas-exposed portion of the gas sensing element;(d) leads connecting with the gas sensing element, extending from thesecond end of the housing for electrical communication with an externaldevice; (e) an insulator retaining therein the leads connecting with thegas sensing element; and (f) a metallic cover having a given length, themetallic cover being joined to the housing to hold the insulator thereinin engagement of an end of the metallic cover to the housing.

[0021] An elastic insulating member is disposed on an end of themetallic cover remote from the housing to retain the leads therein. Ifan outer diameter of the elastic insulating member is defined as E, andan outer diameter of the insulator is defined as F, then E≧F.

[0022] According to the fourth aspect of the invention, there isprovided a gas sensor which comprises: (a) a gas sensing element havingan gas-exposed portion; (b) a hollow housing having a first and a secondend, the housing holding the gas sensing element therein so as toarrange the gas-exposed portion outside the housing for exposure to agas to be measured; (c) a protective cover installed on the first end ofthe housing to cover the gas-exposed portion of the gas sensing element;(d) leads connecting with the gas sensing element, extending from thesecond end of the housing for electrical communication with an externaldevice; (e) an insulator retaining therein the leads connecting with thegas sensing element; and (f) a metallic cover joined to the housing tohold the insulator therein. The hollow housing has an inner chamber andan open end. The open end is crimped to elastically press a metal ring,a sealing member, and an insulating member disposed within the innerchamber to hold the gas sensing element in the inner chamber.

[0023] The housing has formed on the inner chamber a step. The gassensing element has a protrusion which is urged elastically through themetal ring, the sealing member, and the insulating member by crimpingthe open end of the housing into constant engagement with the step ofthe housing.

[0024] The metal ring is made of a given length of a round bar which islooped.

[0025] The gas sensing element is made of a cup-shaped member havingformed therein a chamber and has a platinum-made outer electrode formedon the gas-exposed portion and a platinum-made inner electrode formed onan inner wall thereof. A heater is disposed in the chamber of the gassensing element.

[0026] A spring steel-made outer terminal is electrically connected tothe outer electrode of the gas sensing element. A spring steel-madeinner terminal is electrically connected to the inner electrode. Theouter terminal has a conductive extension which is connected to the endportion of one of the leads within the insulator. The inner terminal hasa conductive extension which is connected to the end portion of theother of the leads within the insulator and a heater-holding portionwhich holds the heater in the chamber of the gas sensing element.

[0027] The heater is formed with a plate heater which has a rectangularcross section and which is made of a lamination of a substrate formedwith a ceramic sheet and a heat generating member.

[0028] The housing has an annular groove formed in the first end and anannular skirt extending from the first end around an outer circumferenceof the annular groove. The protective cover is made of an assembly of anouter cylindrical member and an inner cylindrical member. The outer andinner cylindrical members has flanges which are fitted in the annulargroove of the housing. The annular shirt of the housing is crimpedinward to elastically press the flanges of the outer and innercylindrical members together within the annular groove to join themetallic cover to the housing.

[0029] An elastic insulating member is disposed on an end of themetallic cover remote from the housing to retain the leads therein. Ifan outer diameter of the elastic insulating member is defined as E, andan outer diameter of the insulator is defined as F, then E≧F.

[0030] According to the fifth aspect of the invention, there is provideda production method of a gas sensor including a gas sensing elementhaving an gas-exposed portion, a hollow housing having a first and asecond end portion, holding the gas sensing element therein so as toarrange the gas-exposed portion outside the housing for exposure to agas to be measured, a protective cover installed on the first endportion of the housing to cover the gas-exposed portion of the gassensing element, leads connecting with the gas sensing element,extending from the second end portion of the housing for electricalcommunication with an external device, an insulator retaining thereinthe leads connecting with the gas sensing element, and a metallic coverjoined to the housing to hold the insulator therein. The productionmethod comprises the steps of: (a) preparing an assembly of the housingand the metallic cover attached to an outer wall of the second endportion of the housing; (b) rotating the assembly around a central axisthereof; (c) keeping a rotational speed of the assembly at a givenconstant value; and (d) emitting a laser beam to a circumference of themetallic cover of the assembly to weld the metallic cover to thehousing.

[0031] In the preferred mode of the invention, the assembly is rotatedwith the housing oriented upward and the metallic cover orienteddownward.

[0032] A difference between an outer diameter of the outer wall of thesecond end portion of the housing to be welded to the metallic cover andan inner diameter of the metallic cover, that is, the inner diameter ofthe metallic cover minus the outer diameter of the housing falls withina range of −0.15 mm to 0.1 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be understood more fully from thedetailed description given hereinbelow and from the accompanyingdrawings of the preferred embodiments of the invention, which, however,should not be taken to limit the invention to the specific embodimentsbut are for the purpose of explanation and understanding only.

[0034] In the drawings:

[0035]FIG. 1 is a longitudinal sectional view which shows a gas sensoraccording to the present invention;

[0036]FIG. 2(a) is a side view which shows an insulator;

[0037]FIG. 2(b) is a sectional view taken along the line A-A in FIG.2(a);

[0038]FIG. 2(c) is a sectional view taken along the line B-B in FIG.2(a);

[0039]FIG. 3(a) is a longitudinal sectional view which showsinstallation of an insulator in a cover assembly in a prior artstructure;

[0040]FIG. 3(b) is a longitudinal sectional view which showsinstallation of an insulator in a cover of a gas sensor of theinvention;

[0041]FIG. 4(a) is a plan view which shows an elastic member used inretaining an insulator;

[0042]FIG. 4(b) is a sectional view taken along the line C-C in FIG.4(a);

[0043]FIGS. 5 and 6 are sectional views which show a sequence ofinstallation processes of installing an insulator in a cover;

[0044]FIG. 7 is a partially enlarged view of FIG. 6;

[0045]FIG. 8 is a sectional view which shows an insulator installed in acover in the installation processes shown in FIGS. 5 and 6;

[0046]FIG. 9(a) is a partially sectional view which shows installationof a cover on a housing in a prior art structure;

[0047]FIG. 9(b) is a partially sectional view which shows installationof a cover on a housing in a prior art structure;

[0048]FIG. 10(a) is a perspective view which shows a metal ring used inholding a sensing element;

[0049]FIG. 10(b) is a cross sectional view taken along the line D-D inFIG. 10(a);

[0050]FIG. 11(a) is a perspective view which shows a metal ring used inthe prior art structure as shown in FIG. 9(a);

[0051]FIG. 11(b) is a cross sectional view taken along the line E-E inFIG. 11(a);

[0052]FIG. 12 is a partially sectional view which shows installation ofa cover and a cover assembly on a housing;

[0053]FIG. 13 is a partially sectional view which shows a weld of ahousing and a cover;

[0054]FIG. 14(a) is a perspective view which shows a terminal leading toan outer electrode of a sensing element;

[0055]FIG. 14(b) is a perspective view which shows a terminal leading toan inner electrode of a sensing element;

[0056]FIG. 15(a) is a partially sectional view which shows arrangementof leads in an upper portion of a cover in the prior art structure shownin FIG. 9(a);

[0057]FIG. 15(b) is a partially sectional view which shows arrangementof leads in an upper portion of a cover in a gas sensor of theinvention;

[0058]FIG. 16 is a perspective view which shows a heater disposed in asensing element;

[0059]FIG. 17 is an illustration which shows a welding process ofjoining a cover to a housing;

[0060]FIG. 18(a) is a plan view which shows the first modification of anelastic member used in retaining an insulator;

[0061]FIG. 18(b) is a sectional view taken along the line D-D in FIG.18(a);

[0062]FIG. 19(a) is a plan view which shows the second modification ofan elastic member used in retaining an insulator;

[0063]FIG. 19(b) is a sectional view taken along the line E-E in FIG.19(a);

[0064]FIG. 20(a) is a plan view which shows the third modification of anelastic member used in retaining an insulator;

[0065]FIG. 20(b) is a sectional view taken along the line F-Fin FIG.20(a);

[0066]FIG. 21(a) is a plan view which shows the fourth modification ofan elastic member used in retaining an insulator; and

[0067]FIG. 21(b) is a sectional view taken along the line D-D in FIG.21(a).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] Referring now to the drawings, wherein like numbers refer to likeparts in several views, particularly to FIG. 1, there is shown a gassensor 1 according to the present invention which may be employed inautomotive air-fuel ratio control systems to measure an oxygen contentin exhaust gasses of an internal combustion engine.

[0069] The gas sensor 1 generally includes a sensing element 10 and ahollow cylindrical metallic housing 4 having disposed therein thesensing element 10 hermetically. The housing 4 also serves as a sensormount for mounting the gas sensor 1, for example, in an exhaust pipe ofthe vehicle. The sensing element 10 has a gas-exposed portion 11 exposedto a gas to be measured in a gas chamber 13 defined by a protectivecover assembly 2. The gas sensor 1 also includes leads 81 and 91, aninsulator 5, and a metallic cover 3. The leads 81 and 91 connect withthe sensing element 10 through connectors 75 and 76. The insulator 5 isretained in the metallic cover 3 and holds therein ends of the leads 81and 91.

[0070] A cover 39 is installed on an upper portion of the metallic cover3 through a cylindrical water-repellent filter 57 by crimping. The cover39 and the metallic cover 3 have formed therein first and second airvents 58 and 59 through which the air is introduced into a reference gaschamber 12 defined within the gas sensing element 10 through thewater-repellent filter 57.

[0071] The insulator 5 consists of a hollow cylindrical body 51retaining the leads 81 and 91 in an end thereof and a flange 52 greaterin diameter than the body 51.

[0072] The metallic cover 3 consists of a small-diameter portion 31, alarge-diameter portion 32, and a shoulder portion 33 connecting thesmall-diameter portion 31 and the large-diameter portion 32. Thesmall-diameter portion 31 has the inner diameter which is greater thanthe outer diameter of the cylindrical body 51 and smaller than the outerdiameter of the flange 52. The large-diameter portion 32 has the innerdiameter which is greater than the outer diameter of the flange 52. Themetallic cover 3 is joined at an end directly to the housing 4.

[0073] The insulator 5 is installed in the metallic cover 3 by pressingthe surface 522 of the flange 52 through an elastic member 6 fitted intothe large-diameter portion 32 to bring the surface 521 of the flange 52into constant engagement with the shoulder portion 33. Specifically, theinsulator 5 is retained by the elastic member 6 and the shoulder 33 ofthe metallic cover 3.

[0074] The insulator 5, as clearly shown in FIG. 2(a), includes thecylindrical body 51 and the flange 52 greater in diameter than thecylindrical body 51. The insulator 5 also includes a lower cylindricalportion 53 which is slightly greater in diameter than the cylindricalbody 51. The cylindrical body 51 has, as shown in FIGS. 1 and 2(b),formed therein four holes 511, 512, 513, and 514 through which the leads81 and 91 connecting with the sensing element 10 and a pair of leads 171connecting with a heater 15, as will be described later, pass,respectively. The flange 52 and the lower cylindrical portion 53 have,as shown in FIG. 2(c), formed therein a cylindrical bore 531communicating with the holes 511 to 514.

[0075] The metallic cover 3, as clearly shown in FIGS. 1 and 3(b),consists of the small-diameter portion 31, the large-diameter portion32, and the shoulder portion 33. The small-diameter portion 31 has thesize sufficient for the cylindrical body 51 of the insulator 5 to bedisposed therein. The large-diameter portion 32 has the size sufficientfor the flange 52 of the insulator 5 to be disposed therein.

[0076] The elastic member 6 is, as clearly shown in FIGS. 4(a) and 4(b),an externally serrated lockwasher which consists of an annular plate 61,six lock tabs 62, and six guide tabs 67. The annular pate 61 is smallerthan the inner diameter of the large-diameter portion 32 of the metalliccover 3. The lock tabs 62 project from the outer periphery of theannular plate 61 so as to have the outer diameter greater than the innerdiameter of the large-diameter portion 32. Each of the guide tabs 67 isformed between adjacent two of the lock tabs 62 for facilitating theease of insertion of the elastic member 6 into the metallic cover 3 andincreasing the rigidity of the elastic member 6. It is advisable thatthe number of the lock tabs 62 be greater than or equal to three (3).When the elastic member 6 is installed in the metallic cover 3, it is,as clearly shown in FIG. 1, inserted with the lock tabs 62 all facingtoward an opening of the metallic cover 3.

[0077] The installation of the insulator 5 in the metallic cover 3 isachieved using the elastic member 6 in the following manner.

[0078] First, the elastic member 6 and the insulator 5 are, as shown inFIGS. 5 and 6, set on a jig 69. The jig 69 has a central cylinder 691projecting upward, as viewed in the drawings. The central cylinder 691has formed in an end thereof an annular groove 692 into which the lowercylindrical portion 53 of the insulator 5 is to be fitted. An outer endwall 693 of the central cylinder 691 defining the annular groove 692 is,as can be seen in FIG. 6, designed to have the width so that an end ofthe end wall 693 may come into contact only with the annular plate 61 ofthe elastic member 6 without interfering with the tabs 62 and 67.

[0079] Next, the metallic cover 3 is, as shown in FIGS. 6 and 8, put onthe insulator 5 and the elastic member 6 set on the jig 69 and forced,as can be seen in FIG. 8, into a cylindrical chamber 695 defined by theouter wall of the central cylinder 691 within the jig 69, therebyestablishing elastic tight engagement of the lock tabs 62 of the elasticmember 6 with the inner wall of the metallic cover 3. The lock tabs 62of the elastic member 6 are bent at a given obtuse angle away from thesurface 521 of the insulator 5. This structure causes the elastic member6 to be pressed inward when forced into the large-diameter portion 32 ofthe metallic cover 3, thereby producing the pressure which urges theflange 52 of the insulator 5 into constant engagement with the shoulder33 of the metallic cover 3.

[0080]FIG. 3(a) shows, as one example, installation of the insulator 95in the conventional gas sensor 9. The insulator 95 is substantiallyidentical in structure with the insulator 5 in the embodiment, asdescribed above, and has the flange 952. The insulator 95 is disposedwithin the outer cover 932. The inner cover 931 is fitted partly withinthe outer cover 932 with an end 934 urging the flange 952 of theinsulator 95 into constant engagement with the shoulder 935 of the outercover 932 through the spring 956.

[0081] Comparison between FIGS. 3(a) and 3(b) shows that the use of theelastic member 6 in the gas sensor 1 of the invention eliminates theneed for the inner cover 931 employed in the conventional gas sensor 9and that the length L1 between the shoulder portion 33 and the lower endof the metallic cover 3 may be determined more accurately than thatbetween the shoulder 935 and the lower end of the inner cover 931.

[0082] The elimination of the need for the inner cover 931 allows theinner diameter of the metallic cover 3 to be increased so that thedistance L2, as shown in FIG. 9(b), between the metallic cover 3 and thelead 711 of the signal pickup terminal 71 communicating electricallywith a measuring electrode, as will be described later in detail, of thesensing element 1 0 may be increased as compared with that in theconventional gas sensor 9 shown in FIG. 9(a).

[0083] The housing 4, as shown in FIGS. 1 and 9(b), has a chamber 40, acrimped end 41, and an annular seat 49. The annular seat 49 is formed onan inner wall of the housing 4. The sensing element 10 is retainedwithin the chamber 40 tightly by crimping the end 41 of the housing 4 topress a metallic ring 412, a cylindrical insulator 411, and a sealmember 413 downward, as viewed in the drawings, to urge a central flange109 of the sensing element 10 into constant engagement with the seat 49through a packing 18.

[0084] The conventional gas sensor 9 has, as shown in FIG. 9(a), ahousing 94. The housing 94 has an end 940 crimped to press a metallicring 946 to hold a sensing element 910 within the housing 94 through aninsulator 943, a pad 942, and a seal member 941. Between the metallicring 946 and the insulator 943, a flange 939 of the inner cover 931 isinterposed to secure the inner cover 931 on the housing 94. A packing918 is disposed between the sensing element 910 and an inner wall of thehousing 94.

[0085] As will be apparent from comparison of the structure of the gassensor 1 in FIG. 9(b) with that of the conventional gas sensor 9 in FIG.9(a), the crimped end 41 of the housing 4 is sheathed with the metalliccover 3, thereby resulting in greatly improved corrosion resistance ofthe end 41.

[0086] The conventional gas sensor 9, as discussed above, has the flange939 of the inner cover 931 retained on the seal member 941, the pad 942,and the insulator 943 within the housing 94. The seal member 941 is madeof talc compressed in a production process of the gas sensor 1. Thus, ifthere is a variation in volume of the seal member 941, it will cause thelocation of the flange 939 to be changed, which -results in a change involume of a space within which the metallic ring 946 is to be disposed,thus requiring adjustment of the thickness of the metallic ring 946 forcrimping the end 946 of the housing 94 completely. The change inlocation of the flange 939 will also result in a variation in overalllength of the gas sensor 9. Compensating for this variation requiresadjustment of the spring 956 disposed between the insulator 95 and theouter cover 932. In contrast, the gas sensor 1 of this embodiment hasthe metallic cover 3 bonded directly to the housing 4. The overalllength of the gas sensor 1 is, thus, insensitive to a variation involume of the seal member 413 made of compressed talc, therebyeliminating the need for adjustment of the thickness or size of anyparts.

[0087] The metallic ring 412 of the gas sensor 1 is, as shown in FIGS.10(a) and 10(b), made by looping a given length of a round bar and has agap 418. The metallic ring 941 of the gas sensor 9 is, as shown in FIGS.11(a) and 11(b), made by machining a jointless round strip member havingan oval cross section. This difference in structure allows productioncosts of the metallic ring 412 to be reduced, resulting in a decreasedtotal costs of the gas sensor 1. The use of the metallic ring 412 isrealized with the improvement of airtight sealing established by weldingthe metallic cover 3 to the whole of a circumference of the housing 4.

[0088] The protective cover assembly 2 is, as clearly shown in FIG. 12,retained tightly in an annular groove 420 formed in the bottom of thehousing 4 by crimping an annular extension or skirt 421 inward. Theprotective cover assembly 2 consists of outer and inner cylindricalcovers 21 and 22 which have ends bent outward to form flanges 219 and229, respectively The inner cover 22 has an open end bulged so as to beinstalled in the annular groove 420 in a suitable fit with a guide wall422. The installation of the protective cover assembly 2 on the housing4 is achieved by putting the flanges 219 and 229 in the grooves 420together and bending the skirt 421 inward tightly.

[0089] The metallic cover 3 is, as clearly shown in FIGS. 1 and 12,mounted at an end on the housing 4 in constant contact with an annularstep 48 formed on a flange 64 and joined at a circumferential portion300 to the whole of a periphery of an upper portion of the housing 4 by,for example, laser welding. This structure improves the airtight sealingbetween the housing 4 and the metallic cover 3 as compared with aconventional structure such as the one shown in FIG. 9(a) in which theinner cover 931 is installed at an end thereof in the housing 94.

[0090] The step 48 is machined on an upper surface of the flange 64 tohave a flat surface for facilitating establishment of concentricity ofthe metallic cover 3 and the housing 4 and locating portions of themetallic cover 3 and the housing 4 to be welded to each otheraccurately.

[0091]FIG. 13 shows a cross section of a weld 34 of circumferentialportions of the metallic cover 3 and the housing 4 which were fused bythe laser welding and then solidified. The weld 34 is, as can be seenfrom the drawing, made up of a wider half-moon portion 341 formed in themetallic cover 3 and a narrower semi-oval portion 342 formed in theouter wall of the housing 4. If maximum widths of the half-moon portion341 and the semi-oval portion 342 are defined as A and B, respectively,the depth of the semi-oval portion 342 is defined as D, and thethickness of the metallic cover 3 is defined as T, conditions of B≧0.6Aand D≧T are satisfied. In this embodiment, A is 1.0 mm. B is 0.6 mm. Dis 0.6 mm. T is 0.6 mm. This results in an improved strength of thejoint of the metallic cover 3 and the housing 4.

[0092] The sensing element 10 consists of a cup-shaped solid electrolytebody which has formed therein a chamber 100. Within the chamber 100, theheater 15 is disposed for heating the sensing element 10 up to a givenoperating temperature. The chamber 100 defines a reference gas chamber12 communicating with the air vents 58 and 59. The sensing element 10has outer and inner electrodes 130 and 120 both made of Pt. The outerelectrode 130 is attached to the gas-exposed portion 11 and functions asa measuring electrode, while the inner electrode 120 is attached to aninner wall of the sensing element 10 and functions as a referenceelectrode. In operation, the electromotive force is produced between theouter and inner electrodes 130 and 120 as a function of theconcentration of a gas within the gas chamber 13 and outputted throughthe leads 81 and 91.

[0093] For the operation of the gas sensor 1 in more detail, referenceis made to U.S. application Ser. No. 09/196,693, filed on Nov. 20, 1998,assigned to the same assignee as that of this application, disclosure ofwhich is incorporated herein by reference.

[0094] The signal pickup terminal 71, as shown in FIGS. 1 and 14(a), ismounted on the outer wall of the sensing element 10 in electricalconnection with the outer electrode 130. The signal pickup terminal 72,as shown in FIGS. 1 and 14(b), is fitted in the chamber 100 of thesensing element 10 in electrical connection with the inner electrode 120and has a cylindrical holder 725 which holds the heater 15 therein. Thesignal pickup terminals 71 and 72 have, as already described, the leads711 and 721 electrically connecting with the leads 81 and 91 through theconnectors 75 and 76. The signal pickup terminals 71 and 72 are eachmade of a heat-resisting spring steel such as INCONEL (trade mark) whosemain component is Ni for improving the durability.

[0095] The heater 15, as shown in FIG. 16, includes a ceramic square rod150 having a rectangular cross section. The ceramic square rod 150 ismade of a laminate of substrates each formed with a ceramic sheet andheat generating members. Metallic terminal plates 153 are bonded toopposed surfaces of the ceramic square rod 150 in electrical connectionwith the heat generating members through the ceramic substrates,respectively. The metallic terminal plates 153 each have formed thereonconductive pins 154 connecting with the leads 171.

[0096] A rubber bush 45, as clearly shown in FIG. 15(b), is installed inan end of the small-diameter portion 31 of the metallic cover 3. Therubber bush 45 retains therein the leads 81, 91, and 171. The cover 39is, as described above, installed on the small-diameter portion 31 ofthe metallic cover 3 by crimping. If the outer diameter of the rubberbush 45 is defined as E, and the outer diameter of the cylindrical body51 of the insulator 5 is defined as F, then E≧F. In this embodiment,E=10.1 mm. F=9.8 mm. This structure results substantially in agreementof the interval d between opposed two of four holes in the rubber bush45 through which the leads 81, 91, and 171 pass with the interval Dbetween opposed two of the holes 511 to 514 in the insulator 5.

[0097] In the conventional gas sensor 9 as shown in FIG. 15(a), theouter diameter E of a rubber bush 945 is smaller than the outer diameterF of the insulator 95. Specifically, E is 6.5 mm, and F is 9.8 mm. Thisstructure results in a difference between the intervals d and D whichwill require bending the leads 981 and 991 extending from the insulator95 inward when they are inserted into the rubber bush 945 in anassembling process. It is not advisable that the leads 981 and 991 bebent at sharp angles, thus requiring increase in interval L3 between theinsulator 95 and the rubber bush 945 which will result in increase inoverall size of the gas sensor 9. In contrast, the structure of the gassensor 1 shown in FIG. 15(b) allows the interval L3 between theinsulator 5 and the rubber bush 45 to be decreased, so that the overallsize of the gas sensor 1 can be reduced greatly as compared with the gassensor 9.

[0098] The heater 15 may alternatively be made of a ceramic round bar.The sensing element 10 may alternatively be made of a laminated plateelement. For example, U.S. Pat. No. 5,573,650, issued Nov. 12, 1996 toFukaya et al., teaches such a structure of the sensing element 10,disclosure of which is incorporated herein by reference.

[0099]FIG. 17 shows a process of welding the metallic cover 3 and thehousing 4 together according to the second embodiment of the invention.

[0100] First, the metallic cover 3 is fitted on the housing 4 to form anassembly 100. Next, the assembly 100 is turned about a longitudinalcenter line 101 thereof. After a given rotational speed is reached andkept constant, a welding gun 80 is activated to emit a laser beam 88 tothe circumferential portion 300 of the metallic cover 3.

[0101] Specifically, the outer wall of the housing 4 to be welded to themetallic cover 3 is so machined as to have the diameter greater than theinner diameter of the metallic cover 3 by 0.1 mm. The assembly 100 isformed by pressing the housing 4 into the metallic cover 3.

[0102] The difference between the outer diameter of the outer wall ofthe housing 4 to be welded to the metallic cover 3 and the innerdiameter of the metallic cover 3 (i.e., the inner diameter of themetallic cover 3 minus the outer diameter of the housing 4) may bewithin a range of −0.15 mm to 0.1 mm, preferably within a range of −0.10mm to 0.05 mm for providing for ease of assembly and hermetic sealingbetween the metallic cover 3 and the housing 4.

[0103] Next, a welding jig 85 is provided which consists of a supportingplate 851 and a rotary plate 852. The metallic cover 3 of the assembly100 is installed rotatably in the supporting plate 851, while thehousing 4 is fixed in the rotary plate 852 so that the metallic cover 3may be oriented downward, while the housing 4 may be oriented upward.

[0104] The rotary plate 852 is rotated using, for example, an electricmotor (not shown) to turn the assembly 100. When the circumferentialspeed of the assembly 100 reaches 1500 mm/minute, it is kept constant.The welding gun 80 is turned on to emit the laser beam 88 to join themetallic cover 3 and the housing 4 together.

[0105] The laser beam 88 may be emitted either continuously orintermittently. The circumferential speed of the assembly 100 may be setto another value within a range not sacrificing the welding strength andwelded conditions of the metallic cover 3 and the housing 4.

[0106] FIGS. 18(a) and 18(b) show a modification of the elastic member6.

[0107] The elastic member 6 is different from the one shown in FIGS.4(a) and 4(b) only in that the lock tabs 62 and the guide tabs 67 alllie flush with the annular plate 61. In other words, the elastic member6 has opposed surfaces symmetrical with each other.

[0108] FIGS. 19(a) and 19(b) show the second modification of the elasticmember 6. The elastic member 6 has the lock tabs 62 bent alternately inopposite directions. In this modification, opposed surfaces of theelastic member 6 are, like the above first modification, symmetricalwith each other, which will allow a workman to place the elastic member6 on the jig 69, as shown in FIG. 6, in installation of the insulator 5in the metallic cover 3 without having to pay attention to orientationof the elastic member 6.

[0109] FIGS. 20(a) and 20(b) show the third modification of the elasticmember 6 which is different from the one shown in FIGS. 4(a) and 4(b)only in that the lock tabs 62 are all bent in the same direction at anangle of 45° or more to the surface of the annular plate 61.

[0110] FIGS. 21(a) and 21 (b) show the fourth modification of theelastic member 6 which is different from the one shown in FIGS. 4(a) and4(b) only in that two protrusion 618 are formed on the diametricallyopposed guide tabs 67 as a mark which a workman uses in determiningwhich surface of the elastic member 6 is to be oriented to the inside ofthe metallic cover 3 when installing the insulator 5 in the metalliccover 3.

[0111] Specifically, the protrusions 618 extend in the same direction asthat in which the lock tabs 62 are bent so as not to interfere with theinsulator 5 when installed in the metallic cover 3. The number of theprotrusions 618 is not limited to two (2), and at least one protrusion618 may be provided on the elastic member 6. In either of the third andfourth modifications, it becomes easy for a workman to visually perceiveone of opposed surfaces of the elastic member 6 to be oriented towardthe inside of the metallic cover 3 in installation of the insulator 5.To this end, it is also advisable that either of the opposed surfaces ofthe elastic member 6 be colored or that the opposed surfaces be paintedin different colors.

[0112] While the present invention has been disclosed in terms of thepreferred embodiments in order to facilitate better understandingthereof, it should be appreciated that the invention can be embodied invarious ways without departing from the principle of the invention.Therefore, the invention should be understood to include all possibleembodiments and modifications to the shown embodiments which can beembodied without departing from the principle of the invention as setforth in the appended claims.

What is claimed is:
 1. A gas sensor comprising: a gas sensing elementhaving an gas-exposed portion; a hollow housing having a first and asecond end, said housing holding said gas sensing element therein so asto arrange the gas-exposed portion outside said housing for exposure toa gas to be measured; a protective cover installed on the first end ofsaid housing to cover the gas-exposed portion of said gas sensingelement; leads connecting with said gas sensing element, extending fromthe second end of said housing for electrical communication with anexternal device; a metallic cover installed on the second end of saidhousing, said metallic cover including a small-diameter portion, alarge-diameter portion greater in diameter than the small-diameterportion, and a shoulder portion connecting the smaller-diameter portionand the large-diameter portion; and an insulator including a body and aflange projecting from the body, having disposed therein said leadsconnecting with said gas sensing element, the body having an outerdiameter smaller than an inner diameter of the small-diameter portion ofsaid metallic cover and being disposed within the small-diameterportion, the flange having a first and a second end surface opposed toeach other and an outer diameter which is smaller than an inner diameterof the large-diameter portion of said metallic cover and which isgreater than the inner diameter of the small-diameter portion, saidinsulator being disposed in said metallic cover with the first endsurface of the flange urged by an elastic member to bring the second endsurface into constant engagement with an inner wall of the shoulderportion of said metallic cover.
 2. A gas sensor as set forth in claim 1,wherein said metallic cover has a given length, and wherein the elasticmember is so designed as to produce a first pressure acting on an innerwall of the large-diameter portion of said metallic cover in a radialdirection of the large-diameter portion and a second pressure acting onthe second end surface of the flange of said insulator in a lengthwisedirection of said metallic cover perpendicular to the radial directionof said metallic cover.
 3. A gas sensor as set forth in claim 2, whereinthe elastic member includes an annular plate and tabs, the annular platehaving a diameter smaller than the inner diameter of the large-diameterportion of said metallic cover, the tabs projecting from the annularplate so as to establish elastic engagement with the inner wall of thelarge-diameter portion of said metallic cover.
 4. A gas sensor as setforth in claim 3, wherein the elastic member also include guideprotrusions each of which is disposed between adjacent two of the tabsand which projects from the annular plate to a circular line smallerthan the inner diameter of the large-diameter portion of said metalliccover.
 5. A gas sensor as set forth in claim 3, wherein the elasticmember is made of a plate member having opposed surfaces which aresymmetrical with each other.
 6. A gas sensor as set forth in claim 3,wherein the tabs of the elastic member are so designed that when saidinsulator is inserted into said metallic cover, some of the tabs arebent elastically in a first direction away from one of opposed surfacesof the annular plate by elastic pressure produced by the insertion ofsaid insulator, while the other tabs are bent elastically in a seconddirection opposite the first direction.
 7. A gas sensor as set forth inclaim 3, wherein the tabs extend from the annular plate at an angle ofapproximately 45° or more to one of opposed surfaces of the annularplate.
 8. A gas sensor as set forth in claim 3, wherein the elasticmember also includes a protrusion formed on one of opposed surfacesthereof.
 9. A gas sensor as set forth in claim 3, wherein the elasticmember has opposed surfaces at least one of which is painted so that theopposed surfaces have different colors.
 10. A gas sensor as set forth inclaim 3, wherein the tabs of the elastic member are so designed thatwhen said insulator is inserted into said metallic cover, the tabs arebent elastically in the same direction away from one of opposed surfacesof the annular plate by elastic pressure produced by the insertion ofsaid insulator.
 11. A gas sensor as set forth in claim 1, furthercomprising an elastic insulating member disposed on an end of saidmetallic cover remote from said housing to retain said leads therein,and wherein if an outer diameter of the elastic insulating member isdefined as E, and an outer diameter of said insulator is defined as F,then E≧F.
 12. A gas sensor comprising: a gas sensing element having angas-exposed portion; a hollow housing having a first and a second end,said housing holding said gas sensing element therein so as to arrangethe gas-exposed portion outside the housing for exposure to a gas to bemeasured; a protective cover installed on the first end of said housingto cover the gas-exposed portion of said gas sensing element; leadsconnecting with said gas sensing element, extending from the second endof said housing for electrical communication with an external device; aninsulator retaining therein said leads connecting with said gas sensingelement; and a metallic cover joined directly to said housing to holdsaid insulator therein.
 13. A gas sensor as set forth in claim 12,wherein said housing has an outer wall extending between the first andsecond ends, and wherein said metallic cover is welded to the whole of acircumference of the outer wall of said housing.
 14. A gas sensor as setforth in claim 13, wherein a welded portion is formed with welding ofsaid metallic cover and said housing which includes a wider portionformed in said metallic cover and a narrower portion formed in the outerwall of said housing, and wherein if maximum widths of the wider andnarrower portions are defined as A and B, respectively, the depth of thenarrower portion is defined as D, and the thickness of said metalliccover 3 defined as T, conditions of B≧0.6A and D≧T are satisfied.
 15. Agas sensor as set forth in claim 12, further comprising an elasticinsulating member disposed on an end of said metallic cover remote fromsaid housing to retain said leads therein, and wherein if an outerdiameter of the elastic insulating member is defined as E, and an outerdiameter of said insulator is defined as F, then E≧F.
 16. A gas sensorcomprising: a gas sensing element having an gas-exposed portion; ahollow housing having a first and a second end, said housing holdingsaid gas sensing element therein so as to arrange the gas-exposedportion outside the housing for exposure to a gas to be measured; aprotective cover installed on the first end of said housing to cover thegas-exposed portion of said gas sensing element; leads connecting withsaid gas sensing element, extending from the second end of said housingfor electrical communication with an external device; an insulatorretaining therein said leads connecting with said gas sensing element;and a metallic cover having a given length, said metallic cover beingjoined to said housing to hold said insulator therein in engagement ofan end of said metallic cover to said housing.
 17. A gas sensor as setforth in claim 16, further comprising an elastic insulating memberdisposed on an end of said metallic cover remote from said housing toretain said leads therein, and wherein if an outer diameter of theelastic insulating member is defined as E, and an outer diameter of saidinsulator is defined as F, then E≧F.
 18. A gas sensor comprising: a gassensing element having an gas-exposed portion; a hollow housing having afirst and a second end, said housing holding said gas sensing elementtherein so as to arrange the gas-exposed portion outside the housing forexposure to a gas to be measured; a protective cover installed on thefirst end of said housing to cover the gas-exposed portion of said gassensing element; leads connecting with said gas sensing element,extending from the second end of said housing for electricalcommunication with an external device; an insulator retaining thereinsaid leads connecting with said gas sensing element; and a metalliccover joined to said housing to hold said insulator therein, whereinsaid hollow housing has an inner chamber and an open end, the open endbeing crimped to elastically press a metal ring, a sealing member, andan insulating member disposed within the inner chamber to hold said gassensing element in the inner chamber.
 19. A gas sensor as set forth inclaim 18, wherein said housing has formed on the inner chamber a step,and wherein said gas sensing element has a protrusion which is urgedelastically through the metal ring, the sealing member, and theinsulating member by crimping the open end of said housing into constantengagement with the step of said housing.
 20. A gas sensor as set forthin claim 18, wherein the metal ring is made of a given length of a roundbar which is looped.
 21. A gas sensor as set forth in claim 18, whereinsaid gas sensing element is made of a cup-shaped member having formedtherein a chamber and has a platinum-made outer electrode formed on thegas-exposed portion and a platinum-made inner electrode formed on aninner wall thereof, and further comprising a heater disposed in thechamber of said gas sensing element.
 22. A gas sensor as set forth inclaim 21, further comprising a spring steel-made outer terminalelectrically connected to the outer electrode of said gas sensingelement and a spring steel-made inner terminal electrically connected tothe inner electrode, the outer terminal having a conductive extensionwhich is connected to the end portion of one of said leads within saidinsulator, the inner terminal having a conductive extension which isconnected to the end portion of the other of said leads within saidinsulator and a heater-holding portion which holds said heater in thechamber of said gas sensing element.
 23. A gas sensor as set forth inclaim 21, wherein said heater is formed with a plate heater which has arectangular cross section and which is made of a lamination of asubstrate formed with a ceramic sheet and a heat generating member. 24.A gas sensor as set forth in claim 18, wherein said housing has anannular groove formed in the first end and an annular skirt extendingfrom the first end around an outer circumference of the annular groove,and wherein said protective cover is made of an assembly of an outercylindrical member and an inner cylindrical member, the outer and innercylindrical members having flanges which are fitted in the annulargroove of said housing, the annular shirt of said housing being crimpedinward to elastically press the flanges of the outer and innercylindrical members together within the annular groove to join saidmetallic cover to said housing.
 25. A gas sensor as set forth in claim18, further comprising an elastic insulating member disposed on an endof said metallic cover remote from said housing to retain said leadstherein, and wherein if an outer diameter of the elastic insulatingmember is defined as E, and an outer diameter of said insulator isdefined as F, then E >F.
 26. A production method of a gas sensorincluding a gas sensing element having an gas-exposed portion, a hollowhousing having a first and a second end portion, holding the gas sensingelement therein so as to arrange the gas-exposed portion outside thehousing for exposure to a gas to be measured, a protective coverinstalled on the first end portion of the housing to cover thegas-exposed portion of the gas sensing element, leads connecting withthe gas sensing element, extending from the second end portion of thehousing for electrical communication with an external device, aninsulator retaining therein the leads connecting with the gas sensingelement, and a metallic cover joined to the housing to hold theinsulator therein, said production method comprising the steps of:preparing an assembly of the housing and the metallic cover attached toan outer wall of the second end portion of the housing; rotating saidassembly around a central axis thereof; keeping a rotational speed ofsaid assembly at a given constant value; and emitting a laser beam to acircumference of the metallic cover of said assembly to weld themetallic cover to the housing.
 27. A method as set forth in claim 26,wherein said assembly is rotated with the housing oriented upward andthe metallic cover oriented downward.
 28. A method as set forth in claim26, wherein a difference between an outer diameter of the outer wall ofthe second end portion of the housing to be welded to the metallic coverand an inner diameter of the metallic cover, that is, the inner diameterof the metallic cover minus the outer diameter of the housing fallswithin a range of −0.15 mm to 0.1 mm.